1. Field of the Invention
This invention relates in general to photography and in particular to the manufacture of photographic elements. More specifically, this invention relates to the manufacture of photograhic elements comprised of a support coated on one side with at least one imageforming layer and on the opposite side with separate anticurl and antistatic layers.
2. Description of the Prior Art
Photographic elements typically comprise a support material, such as paper, a polymeric film or polymer-coated paper, having on one side thereof one or more image-forming layers; for example, radiation-sensitive layers comprising a radiation-sensitive agent dispersed in a hydrophilic colloid or an imagereceiving layer comprising a nucleating agent dispersed in a hydrophilic colloid. In a particularly common type of photographic element, the radiation-sensitive agent is a silver halide and the hydrophilic colloid in which it is dispersed is gelatin. Under typical conditions of manufacture and use, photographic elements have a tendency during drying thereof to undergo curling toward the image-forming layer(s). This is a result of the tendency of gelatin and other hydrophilic colloids to shrink on drying, with the resulting creation of tensions within the dried layer(s). To eliminate or reduce the undesirable curling propensity of photographic elements, it is common practice to provide an anticurl layer on the side of the support opposite to the image-forming layer(s). The anticurl layer contains a hydrophilic colloid which shrinks on drying and thereby creates tensions which counteract and balance those produced in the image-forming layer(s). In addition to the hydrophilic colloid, it typically contains a hardening agent, which functions to form a hardened layer that resists removal during photographic processing, and may also contain other materials such as particulate fillers of either organic or inorganic type. Anticurl layers are described, for example, in U.S. Pat. Nos. 2,993,793 and 3,630,742.
In addition to anticurl protection, it is often necessary to provide photographic elements with antistatic protection. The accumulation of static electrical charges on photographic elements is a very serious problem in the photographic art. These charges arise from a variety of factors during the manufacture, handling and use of photographic elements. For example, they can occur on sensitizing equipment and on slitting and spooling equipment, and can arise when the paper or film is unwound from a roll or as a result of contact with transport rollers. The generation of static is affected by the conductivity and moisture content of the photographic material and by the atmospheric conditions under which the material is handled. The degree to which protection against the adverse effects of static is needed is dependent on the nature of the particular photographic element. Thus, elements utilizing high speed emulsions have a particularly acute need for antistatic protection. Accumulation of static charges can cause irregular fog patterns in a photographic emulsion layer, and this is an especially severe problem with high speed emulsions. Static charges are also undesirable because they attract dirt to the photographic element and this can cause repellency spots, desensitization, fog and physical defects.
To overcome the adverse effects resulting from accumulation of static electrical charges, it is conventional practice to include an antistatic layer in photographic elements. Typically, such antistatic layer are composed of materials which dissipate the electrical charge by providing a conducting surface. A very wide variety of antistatic agents are known for use in antistatic layers of photographic elements. For example, U.S. Pat. No. 2,649,374 describes a photographic film comprising an antistatic layer in which the antistatic agent is the sodium salt of a condensation product of formaldehyde and naphthalene sulfonic acid. An antistatic layer comprising an alkali metal salt of a copolymer of styrene and styrylundecanoic acid is disclosed in U.S. Pat. No. 3,033,679. Photographic films having an antistatic layer containing a metal halide, such as sodium chloride or potassium chloride, as the conducting material, a polyvinyl alcohol binder, a hardener, and a matting agent are described in U.S. Pat. No. 3,437,484. In U.S. Pat. No. 3,525,621, the antistatic layer is comprised of colloidal silica and an organic antistatic agent, such as an alkali metal salt of an alkylaryl polyether sulfonate, an alkali metal salt of an arylsulfonic acid, or an alkali metal salt of a polymeric carboxylic acid. Use in an antistatic layer of a combination of an anionic filmforming polyelectrolyte, colloidal silica and a polyalkylene oxide is disclosed in U.S. Pat. No. 3,630,740. In U.S. Pat. No. 3,655,386, the surface conductivity of photographic film is improved by coating it with an aqueous alcohol solution of sodium cellulose sulfate. In U.S. Pat. No. 3,681,070, an antistatic layer is described in which the antistatic agent is a copolymer of styrene and styrene sulfonic acid.
It is known to utilize a single layer which provides both anticurl and antistatic protection to a photographic element. It is also known to use separate anticurl and antistatic layers positioned in contiguous relationship on the support. Use of a single layer which serves both purposes frequently results in a situation in which the layer is not ideal for either purpose. This is because the hydrophilic colloid which is needed to provide curl control is an electrical insulator which adversely affects the electrical conductivity which is needed for antistatic protection, and because the conductive agent which is required for antistatic protection adversely affects the ability of the hydrophilic colloid to counteract and balance the tensions which cause curling. Thus, a coating composition which is optimum for antistatic protection will usually be inferior as regards anticurl protection, and vice versa. This problem is mentioned in U.S. Pat. No. 3,630,742, which points out that use of a single layer of gelatin containing an antistatic agent to provide both anticurl and antistatic protection suffers from serious disadvantages, in that coaction typically occurs between the gelatin and the antistatic agent, with the result that an excessive amount of gelatin is required to reduce curl. As explained in this patent, substantially less gelatin is required to give the desired result when separate anticurl and antistatic layers are provided.
Separate anticurl and antistatic layers can be applied to a photographic element by a simultaneous dual-layer coating procedure; a method which is recommended in U.S. Pat. No. 3,630,742. Equipment and procedures for carrying out such simultaneous coating are well known and are described, for example, in U.S. Pat. Nos. 2,761,417, 2,761,418, 2,761,419, 2,761,791, and 3,508,947. However, in carrying out simultaneous dual-layer coating of anticurl and antistatic layers, a certain degree of inter-layer mixing is generally unavoidable because of the characteristics of the coating compositions. As a result of such inter-layer mixing, excessive amounts of the antistatic agent are needed to achieve the desired low surface resistivity. Since antistatic agents are generally quite costly, the need for high concentrations of antistatic agent is a major disadvantage which seriously hampers the commercial utilization of the process.
The present invention is directed toward the objective of providing a method for the manufacture of a photographic element, provided with separate anticurl and antistatic layers, which is simple to carry out, fast, inexpensive, and capable of making efficient use of both the gelatin, or other hydrophilic colloid, and the antistatic agent, so that both of these materials can be employed at an optimum level.